Method of patterning sub-0.25 lambda line features with high transmission, “attenuated” phase shift masks

ABSTRACT

A method for making a mask for optically transferring a lithographic pattern corresponding to an integrated circuit from the mask onto a semiconductor substrate by use of an optical exposure tool. The method includes the steps of de-composing the existing mask patterns into arrays of “imaging elements.” The imaging elements are π-phase shifted and are separated by non-phase shifting and sub-resolution elements referred to as anti-scattering bars (ASBs). In essence, the ASBs are utilized to de-compose the larger-than-minimum-width mask features to form “halftone-like” imaging patterns. The placement of the ASBs and the width thereof are such that none of the π-phase shifting elements are individually resolvable, but together they form patterns substantially similar to the intended mask features.

This application claims benefit of Prov. No. 60/078,281 filed Mar. 17,1998.

FIELD OF THE INVENTION

The present invention relates to micro-lithography and, in particular,relates to resolution enhancement and proximity correction featuresutilized in the field of photolithography for semiconductor devicefabrication.

BACKGROUND OF THE INVENTION

Until recently, improvements in the resolution of optical lithographyprocesses have come largely from the use of Deep Ultra Violet (DUV)exposure sources having shorter exposure wavelengths (λ). Currentlyavailable DUV exposure sources include the following two types ofexcimer lasers: (1) Krypton Fluoride (KrF) having a λ of 248 nm and (2)Argon Fluoride (ArF) having a λ of 193 nm. However, in order tofabricate device generations at 0.18 μm (180 nm) and below, it hasbecome clear to the industry that the lithography process will need toresolve feature dimensions below λ if either of the two foregoingexposure sources are to be utilized.

An alternative to utilizing the foregoing excimer lasers is non-opticallithography using very short exposure wavelength sources such as ExtremeUltra Violet (EUV), X-ray, or electron beam (E-beam). Unfortunately, allthe non-optical lithography technologies require some form oftechnological break-through combined with an adequate supportinfrastructure in order to become “production-worthy,” or in otherwords, commercially feasible. While it is likely that the necessarytechnology break-through and infrastructure build-up will eventuallyoccur, to date they have not. As such, for semiconductor deviceproduction having design rules in the range of 0.18 μm down to 0.10 μm,optical lithography is currently considered to be the most economicaland preferred process technology. Accordingly, there exists a need tofind innovative methods that can consistently pattern sub-λ devicefeatures with optical lithography.

For sub-λ device features, the mask pattern image formation is stronglydependent on the optical diffraction with the immediately adjacentpatterns. For binary-type masks such as chrome patterns on a quartzglass substrate, the resolution is diffraction limited as imposed by theexposure tool. However, by introducing a π phase shift as the exposurewavefront passes through the mask patterns, it has been demonstratedthat the optical resolution limit can be greatly extended. Depending onthe degree of phase shifting effect on the mask, it is possible todouble the spatial frequency resolution for the mask patterns. In otherwords, the pattern resolution achievable with a phase shift mask (PSM)can reach ½ λ.

The first PSM application in optical lithography was reported by M. D.Levenson in 1982 (IEEE Trans. Electron. Devices 29, 1828, 1982). Sincethis time, there has been a continuous effort in the industry to exploreand develop PSM technology. However, due to the inherent complexity ofmask design, the learning curve for making and applying PSM has beenlong and arduous. Nonetheless, several forms of practical PSM technologyhave been developed. With regard to making line and space patterns,there are three major types: 1) alternating PSM (as originally proposedby Levenson); 2) attenuated PSM; and 3) chromeless PSM.

In accordance with alternating PSM, 0-phase and π-phase alternatingareas are formed between chrome mask features. There are two majorunsettled issues concerning alternating PSM design. The first is theunavoidable conflict of phase assignment, and the second is the unwantedresist patterns caused by the 0 to π phase transitions on the mask. Thecurrently proposed solutions to these issues either add more complexityto the mask design or require the use of more than one mask. As such,none of the proposed solutions are attractive from a “commercialization”or “production cost” point of view.

From a design viewpoint, alternating PSM is much more challenging thanattenuated PSM. Attenuated PSM typically utilizes an energy absorbingthin film layer deposited on a quartz substrate. This energy absorbingfilm has the property of causing a 180 degree (π) phase change in theelectric (E) field as the exposure wavefront passes through the mask.After mask patterns have been delineated, there is a π phase shift inbetween the attenuated film areas and non-patterning glass only areas.Unlike the traditional chrome mask, this type of PSM typically causessome amount of attenuation by the actinic (or effective) exposure λ. Theextent of attenuation is mainly dependent on the phase shifting filmstructures and/or the interlayer thin chrome film deposited on the glasssubstrate. The attenuation permits a certain percent of actinic exposureλ to “leak” through the phase shifting areas of the mask. Normally theamount of attenuation is described as percent transmission (%T).

FIGS. 1A-1C illustrate aerial images of a typical attenuated PSM withintensity profiles (%T) equaling 100%, 25% and 5%, respectively. As canbe observed from FIGS. 1A-1C, relatively high intensity levels resultfrom the attenuated, phase shifted areas. The strength of the intensitylevels seems to be related to the %T. Specifically, the higher the %T,the stronger the intensity level. For the non-phase shifting (glassonly) areas, the intensity levels remains unchanged. In order tominimize these “undesirable” intensity levels resulting from theattenuation, the standard industry practice is to limit the %T to be atmost 5% for DUV exposure λ.

Finally, with regard to chromeless PSM, the π phase shift area can bemade by simply etching into the quartz substrate to a precise depth. Thenon-etched areas and the etched areas have an optical path difference(OPD) that can cause a π phase shift as the exposure wavefront passesthrough the mask. Optically, the chromeless PSM concept is substantiallyan extension of the attenuated PSM. In other words, the chromeless PSMcan be thought of as an attenuated PSM with 100% transmission. Asobserved in FIG. 1A, for a high %T, the “leakage” of actinic exposure λcauses very strong aerial image intensity profiles.

Heretofore, the standard method for controlling the “undesirable”intensity levels is to limit the %T. Unfortunately, very low %T limitsthe potential resolution advantage that can otherwise be gained by usingthe phase shifting film. The lower the %T, the more the resulting filmacts like a non-phase shifting chrome film. Accordingly, in order toachieve higher resolution, it is much more desirable to use a high %Tattenuated PSM. One solution to the foregoing problem is to utilize anopaque film layer to “block” off the leaky phase shifting areas.

As shown in FIG. 2, a chrome opaque film can effectively minimize the“undesirable” intensities. The width of the chrome blocking layer needsto be smaller than the high %T attenuated phase shifting areas. Tomanufacture this chrome blocking layer, it is necessary to perform asecond resist coating, alignment, and imaging process. This second steprequires tight control of the width of the chrome blocking layer and thealignment margin in order to ensure the chrome blocking layer will beeffective and not interfere with the phase shifting pattern areas.

It is clear that one disadvantage of using a chrome blocking layer isthe need to perform two alignment processes for making such a reticle.The chrome blocking layer is normally imaged by an optical laser patterngenerator. As such, it often suffers from lower resolution and limitedalignment accuracy. In addition, this second process step adds to boththe complexity and cost of the mask.

Moreover, as stated, the chrome blocking layer is utilized to “block”the “undesirable” aerial image intensities formed by the high %T phaseshifting areas. As shown in FIG. 2, the remaining aerial images aremainly formed by the non-phase shifting patterns. However, the aerialimage intensity levels are not as high as the ones formed by the high %Tphase shifting areas. As a result, the expected resolution enhancementfrom the traditional chrome-blocked PSM is substantially negated.

Accordingly, there remains a need for a mask which allows for the use ofthe high intensity levels formed by the high %T phase shifting areas(because the higher intensity levels offer an inherent higher resolutionpotential), and which does not require the use of chrome blocking layersso as to reduce the overall complexity and cost of the mask.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a costeffective and practical method for patterning sub-0.25λ resist linefeatures using a type of 100% transmission, “attenuated” PSM.

More specifically, the present invention relates to a method for makinga mask for optically transferring a lithographic pattern correspondingto an integrated circuit from the mask onto a semiconductor substrate byuse of an optical exposure tool. The method comprises the steps ofde-composing the existing mask patterns into arrays of “imagingelements.” These imaging elements are π-phase shifted and are separatedby a non-phase shifting and sub-resolution element referred to asanti-scattering bars (ASB). In essence, the ASBs are utilized tode-compose the larger-than-minimum-width mask features to form“halftone-like imaging patterns. The placement of the ASBs and the widththereof are such that none of the π-phase shifting elements areindividually resolvable, but together they form patterns substantiallysimilar to the intended mask features. The isolated minimum width linefeatures can be formed by a single π-phase imaging element.

As described in detail below, the method of the present inventionprovides important advantages over the prior art. Most importantly, thepresent invention discloses a method for patterning sub-0.25λ resistline features using a type of 100% transmission, “attenuated” PSM. Inaccordance with the present invention, instead of trying to eliminatethe image intensity caused by high transmission π-phase pattern areas,the method of the present invention makes use of the high contrastaerial image to achieve excellent printing resolution.

In addition, by extending the concept of ASBs, it is possible to“decompose” the π-phase feature patterns. Using the decomposed π-phaseshifting elements, it is possible to reconstruct the random shapeddevice patterns, while simultaneously performing optical proximitycorrection by manipulating the size, shape, and placement of thedecomposed π-phase shifting elements.

Furthermore, as the imaging concept for the high transmission,attenuated PSM method of the present invention is very similar to theconventional, non-phase shifting chrome mask patterning methods, it isbelieved that the adoption of this technology by the industry will bemuch easier as compared to alternating PSM technology. From the masklayout point of view, by utilizing the method of the present inventionthere is no need to be concerned with avoiding phase conflicts andprinting of phase transitions onto the wafer. Thus, the mask layoutcomplexity is greatly reduced. Moreover, as there is no need to use anopaque chrome blocking layer, the mask making process is much simpler.

The method of present invention also provides for the decomposition of aminimum line feature into an array of π-phase shifting elements so thatit is possible to use a wider dimension element on the 4× mask toachieve printing 0.25λ feature on a wafer. As a result, the mask usedfor printing the sub-0.25λ features can be made at a reasonable cost.

Additional advantages of the present invention will become apparent tothose skilled in the art from the following detailed description ofexemplary embodiments of the present invention.

The invention itself, together with further objects and advantages, canbe better understood by reference to the following detailed descriptionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate aerial images of a typical attenuated PSM withintensity profiles (%T) equaling 100%, 25% and 5%, respectively.

FIG. 2 illustrates an aerial image of a typical attenuated PSM utilizingan opaque chrome film layer.

FIGS. 3A-3C are aerial images illustrating that as the width ofnon-phase shifting features becomes smaller, the intensity levels fornon-phase shifting features tend toward zero.

FIG. 4A illustrates a prior art layout for a 1 μm feature.

FIG. 4B illustrates the aerial image for the 1 μm feature of FIG. 4Autilizing 100%T attenuated PSM.

FIG. 4C illustrates an exemplary layout for a 1 μm feature “halftoned”utilizing ASBs in accordance with the present invention.

FIG. 4D illustrates the aerial image for the 1 μm feature of FIG. 4Cutilizing 100%T attenuated PSM.

FIG. 5A illustrates the use of the present invention to form maskpatterns for randomly shaped logic gate features to be utilized with100%T attenuated (chromeless) PSM.

FIG. 5B illustrates the resulting printed resist patterns formed fromutilizing the mask patterns of FIG. 5A.

FIG. 6 illustrates an example of how OPE varies in accordance with ASBwidth.

FIG. 7 illustrates an exemplary method of de-composing the minimum linefeature into an array of π-phase shifting elements in accordance withthe present invention.

FIG. 8 illustrates a comparison of aerial images for the “halftoned” and“non-halftoned” line features set forth in FIG. 6, as well as the aerialimage of a non-halftoned 0.40 μm line feature.

FIG. 9 illustrates an exemplary mask layout comprising dual chrome-SBfor DOF improvement.

FIG. 10 illustrates a comparison between aerial images for on-axisversus off-axis (annular) illumination.

FIG. 11 illustrates the impact on aerial image contrast by maximumillumination setting.

DETAILED DESCRIPTION

The following detailed description relates to a novel method forpatterning sub-0.25λ resist features using a type of 100% transmission,“attenuated” PSM. As explained below, the novel method utilizes opticalproximity correction (OPC) assist features to modulate the intensitylevels in the phase shifting areas. As a result of the presentinvention, all the mask feature patterning can be done in one pass by ahigh resolution electron beam mask writer, thereby eliminating the needfor use of chrome blocking layers.

In FIGS. 1A-1C, it was shown that for a certain width of non-phaseshifting features, the strength of the aerial image intensity peaks wasusable. However, as the width of non-phase shifting features becomessmaller (i.e., as the two phase shifting features move closer together),the intensity levels for non-phase shifting features become almost zerofor 100%T, as shown in FIG. 3A-3C.

As also shown in FIGS. 3A-3C, the intensity levels for the phaseshifting features also decreased. At lower %T, the negative impact onthe intensity levels of the phase shifting features becomes much moreapparent and the intensity levels for the non-phase shifting featuresrise slightly. The net effect is an overall decrease of aerial imagecontrast for the phase shifting features. It is believed that theforegoing negative impact on the intensity levels is caused by opticalproximity effects (OPE). In order to eliminate the foregoing negativeeffects resulting from the OPE, the present invention utilizes a novelsub-resolution clear OPC assist feature to control the intensity levelsfor the large phase shifting width features.

More specifically, when forming the mask in accordance with the presentinvention, each feature having a width greater than 1.22(λ/NAo) is“halftoned” utilizing anti-scattering bars (ASB). Anti-scattering bars,which are described in U.S. Pat. No. 5,447,810, are clear sub-resolutionassist features. FIGS. 4A-4D illustrate a comparison between prior artfeature patterning and the “halftoning” technique of the presentinvention.

FIG. 4A illustrates an exemplary (prior art) layout for a 1 μm feature12. FIG. 4B illustrates the aerial image for the 1 μm feature 12utilizing 100%T attenuated PSM. As shown in FIG. 4B, the phase shiftingarea 14 associated with the feature 12 exhibits a substantial intensitylevel. As stated above, heretofore, such intensity levels were blockedby utilizing an additional chrome blocking layer.

FIG. 4C illustrates the present invention which eliminates the effectsof OPE without the need of a chrome blocking layer. As shown, inaccordance with the present invention, the 1 μm feature is “halftoned”utilizing ASBs 16. In other words, the feature is formed such thatπ-phase shifting elements 18 are separated by ASBs 16 (i.e., the π-phaseshifting elements are “halftoned”). Referring to FIG. 4D, the resultingaerial image for the “halftoned” feature illustrates the elimination ofthe unwanted intensity levels, even though 100%T attenuated PSM isutilized.

It has been determined that the preferred dimensions of the π-phaseshifting elements 18 and the ASBs 16 are as follows:

a) the width (W1) of each π-phase feature is preferably between 0.20 to0.35(λ/NAo), and

b) the width (W2) of each ASB feature is preferably not greater than0.35(λ/NAo), where NAo is the numerical aperture for the objective lensof the exposure source. In addition, the halftone period (HTP) of the“halftoned” feature, which is defined as the distance between the edgeof one π-phase shifting elements 18 to the same edge of an adjacentπ-phase shifting elements 18, must be kept sub-resolution such that theindividual elements are non-resolvable by the optical exposure toolutilized in the photolithography process. It is noted that in FIG. 4C,the halftone period equals W1+W2. In order for the halftone period to bekept sub-resolution, in accordance with Rayleigh's criterion, thefollowing equation must be satisfied:

HTP<k1 (λ/NAo),

where k1=0.61, λ equals the wavelength of the optical exposure source,and NAo equals the numerical aperture of the objective lens of theexposure source.

The “halftoning” concept of the present invention, which essentiallyplaces ASBs, or “gaps” between π-phase shifting elements, can also beapplied to features having random shapes. More specifically, utilizingthe present invention it is possible to decompose random features into aseries of π-phase shifting elements. In the case of a clear-field,chromeless (100%T attenuated) PSM, the main circuit features are formedby “π-phase” elements on the mask. It is noted that this technique isvery similar to the traditional technique for forming chrome mainfeatures on glass. Because of this similarity, it is believed that itwill be relatively easy for the industry to adopt this style of PSM asopposed to alternating PSM. Most importantly, there is no phase conflictproblem, and therefore the PSM design complexity is greatly reduced.Moreover, as the 0 to π phase transition is used for patterndelineation, it is possible to obtain very high resolution.

FIG. 5A illustrates the use of the present invention to form maskpatterns for randomly shaped logic gate features with 100%T attenuated(chromeless) PSM. As shown, both of the features 21, 22 are decomposedinto a series of π-phase shifting elements 18 on the mask. FIG. 5Billustrates the resulting printed resist patterns 23, 24 (i.e., thesolid dark line in FIG. 5B). Specifically, as shown in FIG. 5B, a 0.103πm gate feature 23 is well printed, and the feature width is only 40% ofthe KrF exposure λ.

As indicated above, the 100%T attenuated PSM typically exhibits a strongOPE in the corners and line-ends of the feature. This OPE can beeffectively compensated for by manipulating the shape, dimension, andplacement of the π-phase shifting elements 18 forming the feature.Referring again to FIGS. 5A and 5B, there are two examples of formingrandomly shaped gate device features. Applicants have discovered that inthe strong electric field area, such as corners, a shorter/smallerπ-phase shifting element 18 should be utilized. With a sophisticated OPCalgorithm, a comprehensive OPC treatment can be made possible forsub-half wavelength patterning with this style of PSM technology.

More specifically, it was found that the OPE mentioned above can beexpressed in terms of minimum intensity level of the π-phase shiftingelements as a function of ASB width (i.e., the gap between the π-phaseshifting elements). This is illustrated in FIG. 6. When printing aresist line feature with a positive photoresist process, such as theinstance case, the minimum intensity level can be related to the resistfeature width. Within the range of halftone period (0.24 μm), FIG. 6illustrates an example of the minimum aerial image intensity of a 0.06μm width π-phase shifting element being clearly dependent on the ASBwidth (which ranges from 0.06 μm up to 0.18 μm). In other words, the OPEcan be modulated by the ASB width (or simply the separation between theedges of the π-phase shifting elements). As the OPE can be predeterminedeither by simulation or actual wafer printing experiment, it is possibleto use a best-fit ASB width to correct for the OPE at desirablelocations of the mask patterns.

A design guideline for decomposing the main features into π-phaseshifting elements is as follows. The optimum π-phase shifting elementdimension is preferably between 0.20 to 0.35(λ/NAo). It was determinedthat it is preferably for the ASB width to be the same as the π-phaseshifting element and be less than 0.35(λ/NAo). However, as explainedabove, the ASB width is adjusted to correct for OPE. It is noted that intheory, the π-phase shifting element should be as small as possible suchthat a much finer OPC scheme could be implemented. However, givencurrent mask production limitations, a minimum dimension of 0.20(λ/NAo)is believed to be manufacturable on a 4× phase shift mask. For a 0.57NAstepper with KrF exposure λ, the minimum 4× mask dimension isapproximately 0.35 μm. This size feature is routinely formed utilizingcurrently known advanced OPC binary chrome masks.

One of the main limitations for printing near 0.25λ features with 100%Tattenuated (chromeless) PSM is mask process resolution. To achieveprinting of 0.25λ feature with a KrF exposure tool, the mask feature ona typical 4× DUV mask is going to be approximately 0.25 μm. While thismay be attainable, it would be difficult and probably carry a yieldpenalty. For a production-worthy mask making process, it is easier ifthe mask feature can be made as large as possible while still smallenough to correctly print 0.25λ resist features on wafer.

However, by de-composing the minimum line feature into an array ofπ-phase shifting elements in accordance with the present invention, itis possible to use a wider dimension element on the 4× mask to achieveprinting 0.25λ feature on wafer. An example of such a mask layout isshown in FIG. 7.

As shown in FIG. 7, the original line feature 31 to be printed is a 0.26μm line CD. In accordance with the present invention, the line featureis decomposed into an array of π-phase shifting elements 18 so as toform a “halftoned” line feature. Each π-phase shifting element 18 has a0.4 μm mask CD, which is considerably larger than the original 0.26 μmline CD, and is therefore printed more easily. The π-phase shiftingelement array also satisfies the HTP requirement noted above. It hasbeen determined that carefully-tuned, halftone line array elements canproduce the same aerial image as one with a smaller line feature, as isillustrated in FIG. 8.

It has been determined that the aerial image profile of a halftone lineπ-phase array can be manipulated by adjusting the ASB width (or theseparation between the edges of the two adjacent π-phase shiftingelements) and the dimensions of the π-phase shifting elements. For thesame aerial image profile, either larger π-phase shifting elements withwider ASB width or smaller π-phase shifting elements with narrower ASBwidth can be utilized. As the result of various experiments, it wasdiscovered that it is permissible to increase the dimension of theπ-phase shifting element by as much as 50%. This should be done withinthe range of the corresponding halftone period. As a result of thismethod, it is possible to make the halftone array have the same aerialimage profile as one with a smaller non-halftone π-phase feature.

FIG. 8 illustrates a comparison of aerial images for the “halftoned” and“non-halftoned” line features set forth in FIG. 7, as well as the aerialimage of a non-halftoned 0.40 μm line feature. As shown in FIG. 8, the0.4 μm halftoned line feature prints with a nearly identical resist CDto that of a 0.26 μm non-halftone line feature, because the two featureshave almost the same aerial image profile.

The halftone duty cycle of the halftone line feature shown in FIG. 7 isapproximately 67%. The halftone duty cycle (%H) equals (d/HTP)*100. HTPin the example shown in FIG. 7 is equal to D+S, where D is the length ofa π-phase shifting element, and S is the spacing between π-phaseshifting elements 18. By manipulating the halftone period to obtain a50% duty cycle, it is possible to obtain an aerial image that isequivalent to an even smaller non-halftone mask line width. Thus, thisnovel method permits the use of readily manufacturable mask feature CDs(0.40 μm on 4× mask) to print resist features that are smaller than0.25λ.

When printing such small features as shown for example in FIG. 7,another major concern is maintaining depth of focus performance. It hasbeen discovered that the use of chrome scattering bars (SB) helpsmaintain resist CD throughout the focus window. FIG. 9 is an exemplarymask layout illustrating the application of dual chrome-SB to assist thehalftone a-phase line feature.

As shown in FIG. 9, the SBs 41 utilized in the present example have a CDof 0.24 μm on the 4× mask. Since this is a non-phase shifting feature,the SBs 41 are well below the printable resolution. The main function ofthe dual SB 41 is to help maintain the log-slope of the aerial image forthe “isolated” halftone π-phase shifting elements 18 while in a de-focuscondition. Table 1 sets forth CD data obtained via a simulationutilizing a KrF exposure with 0.57 NAo, 0.75 σ, and a resist thicknessof 0.42 μm (a represents the partial coherence ratio NAc/NAo, where NAcequals the numerical aperture of the illumination condenser.

TABLE 1 4X Mask CD 0.40 μm Halftone 0.40 μm Halftone 0.26 μm Line OPCwith dual SB No SB No SB Focus (μm) Resist CD (μm) Resist CD (μm) ResistCD (μm) −0.5 0.039 −0.4 0.057 0.045 0.049 −0.3 0.067 0.064 0.067 −0.20.070 0.071 0.072 −0.1 0.069 0.068 0.070 0 0.062 0.053 0.059 +0.1 0.0460.020 0.030 +0.2 0.020 Exposure Used 15.6 mJ/cm² 14.4 mJ/cm² 14.4 mJ/cm²Estimated DOF ≈0.45 μm ≈0.30 μm ≈0.30 μm

From the data set forth in Table 1, it is clear that the use of SBs 41does improve the DOF. However, even without the SB 41, the DOFperformance is still very impressive at less than 0.25λ, resist linewidth.

It is noted that while the use of chrome SBs 41 can improve DOF, theimplementation of chrome SBs on a “chromeless” PSM requires anadditional mask process step.

The re-alignment accuracy of the second mask making process can be animportant factor to consider for deploying the SB. Empirically, there-alignment accuracy needs to be +/−50 nm (for 4× reticle) or better inorder to ensure the effectiveness of the SB. Fortunately, suchre-alignment accuracy is achievable by current mask making processes.The use of SBs appears most helpful when attempting to print at near orbelow λ/4 features, as indicated in Table 1. In this instance, theapplication of the SB is mainly to assist the more isolated π-phasefeatures (single element or halftoned), or for π-phasefeature-to-feature spaces that have sufficient room to insert a SB toattain the benefit.

It has been empirically determined that the placement of a single SB(i.e., one SB per each side of the halftone π-phase feature) is afunction of λ and NAo, and the following equation has been derived:

SB edge separation to the halftone π-phase feature edge=k_(s)(λ/NAo),

where k_(s) ranges from 0.55 to 0.70.

k_(s) is affected by the illuminator setting. For more incoherentillumination (σ>0.60) or off-axis illumination such as annular orquadrupole types, it is more effective to use 0.55 to 0.63 for k_(s).Alternatively, for σ<0.60, it is more beneficial to use k_(s) closer to0.63 to 0.70. As an example, for a KrF exposure tool with 0.61 NA, withan illumination setting at 0.75σ, the ideal SB placement would be {tildeover (=)}0.24 μm (or {tilde over (=)}0.96 gm on 4× reticle) away fromthe halftone π-phase shifting feature edge. Placing more than one SB pereach side of the halftone π-phase shifting feature can further improveDOF. The second SB is preferably placed approximately 1.05 to 1.2(λ/NAo) away from the edge of the of the halftone π-phase shiftingfeature. Using the same exposure tool example, the second SB ispreferably placed at 0.18 μm (or 0.72 μm on 4× reticle) away from thenearest edge of the first SB. The placement of a third (and more) SB isless critical. For example, the third SB can be placed apart from thesecond SB utilizing the same amount of separation as between the firstSB and the second SB.

It has also been empirically determined that the width of a single SBequals:

SB width=k_(w)(λ/NAo),

where k_(w) ranges from 0.20 to 0.25.

k_(w) is mainly affected by the contrast of the resist process. For ahigher contrast resist process, it is possible to use kw near 0.25 (orwider SB width). For the current state of the art KrF resist process,the typical SB width is 0.08 μm (or {tilde over (=)}0.32 μm on 4×reticle). For an ArF resist process, the SB is expected to be 0.06 μm(or {tilde over (=)}0.24 μm on 4× reticle).

Another limitation with regard to printing sub-0.25λ resist line width,is the minimum feature pitch. It has been determined that the smallestfeature-to-feature space that could be used for this technology islimited to approximately 0.70(λ/NAo). Below this feature-to-featurespace range, the aerial image contrast becomes too low to form printableresist patterns. For example, for a 0.57NA stepper and KrF exposure, theminimum feature-to-feature space printable is about 0.30 μm. Forprinting a 0.05 μm line feature, the minimum line/space ratio is nearly1:6.

Utilizing a smaller λ and larger NAo clearly improves the aerial imagecontrast. However, for a given λ and NAo, the aerial image contrast canalso be improved by utilizing off-axis illumination. FIG. 10demonstrates this effect.

FIG. 10 illustrates a comparison between aerial images for on-axisversus off-axis (annular) illumination. The π-phase line feature is 0.05μm and the space is 0.25 μm, both dimensions are in 1× wafer scale. Itwas found that with an optimized off-axis illumination, such as the oneshown in FIG. 10, it is possible to shrink the printablefeature-to-feature space to be slightly less than 0.60(λ/NAo). For thisoff-axis illumination case, it is believed that the printable featurespace is at near KrF λ or 0.248 μm. This reduces the printable line tospace ratio to 1:5.

The use of OAI is more beneficial for the circuit designs that requiresmaller feature pitch. The decision to use OAI is essentially determinedby the need for minimum feature pitch. For example, if the currentdesign calls for 0.06 μm circuit width with a minimum feature pitch of0.30 μm, then the use of OAI for the printing process is helpful. Theforegoing is illustrated in FIG. 11. Specifically, FIG. 11 plots theaerial image contrast of the 0.3 μm pitch feature against the maximumillumination σ setting. The maximum illumination σ refers to σ(c) or σ1,as indicated in FIG. 10.

As shown in FIG. 11, a higher illumination a (i.e., a higher NAc)improves aerial image contrast. For the conventional on-axisillumination, the aerial image contrasts of such features never reach0.5 when the ratio of NAc and NAo becomes unity (maximum σ of 1). Anaerial image contrast of 0.5 or higher has been considered to beminimally resolvable. If OAI is utilized, such as the annular typesshown in FIG. 11, the aerial image contrasts can be improved to above0.5 for annular widths from 0.2 to 0.4. The annular width is defined asthe delta between σ1 and σ2 in FIG. 10. In FIG. 11, it is also shownthat the aerial image contrast is more influenced by the maximumillumination σ and less affected by the annular width. Since the widerannular width permits more “light” to go through, the necessary exposuretime is reduced and more throughput is provided. Based on this, theoptimum illumination setting is preferably set to maximum σ in between0.8 and 0.9 with annular width slightly less than 0.4.

FIG. 11 illustrates that OAI works well with halftone π-phase shiftingfeatures at effective λ/4 feature width. It is believed that OAI is morea function of feature pitch and less concerned with feature width. Whilenot shown in FIG. 11, it is noted that the quadrupole type of OAI willalso work well. This is due to the fact that both are off-axisillumination with obstructions in the center of the illumination. Thesame method, as described above, can be utilized to optimize thequadrupole illumination.

It is noted that the present invention is intended to be applicable tohigh transmission “attenuated” phase shift masks. The term “hightransmission” encompasses a % transmission ranging from 6% to 100%.

As described above, the method of the present invention providesimportant advantages over the prior art. Most importantly, the presentinvention discloses a method for patterning sub-0.25λ resist linefeatures using a type of 100% transmission, “attenuated” PSM. Inaccordance with the present invention, instead of trying to eliminatethe image intensity caused by high transmission π-phase pattern areas,the method of the present invention makes use of the high contrastaerial image to achieve excellent printing resolution.

In addition, by extending the concept of ASB, it is possible to“decompose” the π-phase feature patterns. Using the decomposed π-phaseshifting element, it is possible to reconstruct the random shaped devicepatterns, while simultaneously performing optical proximity correctionby manipulating the size, shape, and placement of the decomposed π-phaseshifting elements.

Furthermore, as the imaging concept for the style of the hightransmission, attenuated PSM method of the present invention is verysimilar to the conventional, non-phase shifting chrome mask patterningmethods, it is believed that the adoption of this technology by theindustry will be much easier as compared to alternating PSM technology.From the mask layout point of view, utilizing the method of the presentinvention there is no need to be concerned with avoiding phase conflictsand printing of phase transitions onto the wafer. Thus, the mask layoutcomplexity is greatly reduced. Moreover, as there is no need to use anopaque chrome blocking layer, the mask making process is much simpler.

The method of present invention also provides for the decomposition of aminimum line feature into an array of π-phase shifting elements so thatit is possible to use a wider dimension element on the 4× mask toachieve printing 0.25λ feature on a wafer. As a result, the mask usedfor printing the sub-0.25λ features can be made at a reasonable cost.

Although certain specific embodiments of the present invention have beendisclosed, it is noted that the present invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The present embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims ratherthan the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A method for making a mask for opticallytransferring a lithographic pattern corresponding to an integratedcircuit from said mask onto a semiconductor substrate by use of anoptical exposure tool, said method comprising the steps of: creatingsaid mask having said lithographic pattern corresponding to saidintegrated circuit, said mask having a plurality of features to beprinted; and de-composing each of said plurality of features having awidth exceeding a predetermined value into a plurality of π-phaseshifting elements, each of said plurality of π-phase shifting elementsbeing separated from one another by a non-phase shifting element,wherein the distance between each of said plurality of π-phase shiftingelements is such that none of the individual π-phase shifting elementsare resolvable.
 2. The method of claim 1, wherein each of said pluralityof features having a width less than said predetermined value isimplemented with a single π-phase shifting element.
 3. The method ofclaim 1, wherein the space between each of said plurality of π-phaseshifting elements comprises a non-phase shifting material.
 4. The methodof claim 1, wherein each of said plurality of π-phase shifting elementshas a width between 0.20*(λ/NAo) and 0.35*(λ/NAo), where λ equals thewavelength of the light source emitted by said optical exposure tool,and NAo equals the numerical aperture of an objective lens of saidoptical exposure tool.
 5. The method of claim 4, wherein each of saidnon-phase shifting elements has a width not greater than 0.35*(λ/NAo).6. The method of claim 1, wherein said predetermined value is1.22(λ/NAo).
 7. The method of claim 4, wherein each of said plurality ofπ-phase shifting elements are equally spaced from one another, and eachof said plurality of π-phase shifting elements have the same width. 8.The method of claim 7, wherein said distance between each of saidplurality of π-phase shifting elements and the width of each of saidplurality of π-phase shifting elements define a halftone period, saidhalftone period being less than 0.61*(λ/NAo).
 9. The method of claim 1,wherein said non-phase shifting element is transparent.
 10. The methodof claim 1, further comprising placing scattering bars adjacent to anyisolated π-phase shifting elements, each of said scattering bars havinga width sufficiently narrow such that none of the individual scatteringbars are resolvable.
 11. A method for making a mask for opticallytransferring a lithographic pattern corresponding to an integratedcircuit from said mask onto a semiconductor substrate by use of anoptical exposure tool, said method comprising the steps of: creatingsaid mask having said lithographic pattern corresponding to saidintegrated circuit, said mask having at least one feature to be printed,said feature having a defined width; and de-composing said feature intoa plurality of π-phase shifting elements, each of said plurality ofπ-phase shifting elements being separated from one another by anon-phase shifting element, each of said π-phase shifting elementshaving a width greater than the width of said feature, wherein thedistance between each of said plurality of π-phase shifting elements issuch that none of the individual π-phase shifting elements areresolvable.
 12. The method of claim 11, wherein each of said pluralityof π-phase shifting elements has a width w1, and a separation w2, saidplurality of π-phase shifting elements defining a critical halftoneperiod which equals w1+w2.
 13. The method of claim 12, wherein said maskis utilized in a photolithography process comprising an exposure sourcehaving an associated wavelength λ and an objective lens having anassociated numerical aperture NAo.
 14. The method of claim 13, whereinsaid critical halftone period of said non-resolvable π-phase shiftingelements is less than 0.61 multiplied by (λ/NAo).
 15. The method ofclaim 11, further comprising placing scattering bars adjacent anyisolated π-phase shifting elements, each of said scattering bars havinga width sufficiently narrow such that none of the individual scatteringbars are resolvable.
 16. The method of claim 15, wherein the distancefrom a scattering bar to an isolated π-phase shifting element equalsK_(s)(λ/NAo), λ equaling the wavelength of the exposure source, NAoequaling the numerical aperture of the objective lens of the exposuresource, and K_(s) ranging from 0.55 to 0.70.
 17. The method of claim 15,wherein the width of each scattering bar equals K_(w)(λ/NAo), λ equalingthe wavelength of the exposure source, NAo equaling the numericalaperture of the objective lens of the exposure source, and K_(w) rangingfrom 0.20 to 0.25.
 18. A photolithography mask for opticallytransferring a lithographic pattern corresponding to an integratedcircuit from said mask onto a semiconductor substrate by use of anoptical exposure tool, said mask comprising: a plurality of featurescorresponding to elements of said integrated circuit to be printed; eachof said plurality of features having a width exceeding a predeterminedvalue comprising a plurality of π-phase shifting elements, each of saidplurality of π-phase shifting elements being separated from one anotherby a non-phase shifting element, wherein the distance between each ofsaid plurality of π-phase shifting elements is such that none of theindividual π-phase shifting elements are resolvable.
 19. Thephotolithography mask of claim 18, wherein each of said plurality ofπ-phase shifting elements has a width between 0.20*(λ/NAo) and0.35*(λ/NAo), where λ equals the wavelength of the light source emittedby said optical exposure tool, and NAo equals the numerical aperture ofan objective lens of said optical exposure tool.
 20. Thephotolithography mask of claim 19, wherein said non-phase shiftingelement has a width not greater than 0.35*(λ/NAo).
 21. Thephotolithography mask of claim 19, wherein said predetermined value is1.22(λ/NAo).
 22. The photolithography mask of claim 19, wherein each ofsaid plurality of π-phase shifting elements are equally spaced from oneanother by a predetermined distance, and each of said plurality ofπ-phase shifting elements have the same width.
 23. The photolithographymask of claim 22, wherein said distance between each of said pluralityof π-phase shifting elements and the width of each of said plurality ofπ-phase shifting define a halftone period of said plurality of π-phaseshifting elements, said halftone period being less than 0.61*(λ/NAo).24. The photolithography mask of claim 18, wherein said non-phaseshifting element is transparent.
 25. The photolithography mask of claim18, further comprising placing scattering bars adjacent any isolatedπ-phase shifting elements, each of said scattering bars having a widthsufficiently narrow such that none of the individual scattering bars areresolvable.
 26. A photolithography mask for optically transferring alithographic pattern corresponding to an integrated circuit from saidmask onto a semiconductor substrate by use of an optical exposure tool,said mask comprising: a plurality of π-phase shifting elementscorresponding to a feature to be printed, said feature to be printedhaving a defined width, and a plurality of non-phase shifting elements,each of said plurality of π-phase shifting elements being separated fromone another by one of said non-phase shifting elements, wherein each ofsaid π-phase shifting elements has a width greater than the width ofsaid feature to be printed, and the distance between each of saidplurality of π-phase shifting elements is such that none of theindividual π-phase shifting elements are resolvable.
 27. Thlephotolithography mask of claim 26, wherein each of said plurality ofπ-phase shifting elements has a width w1, and a separation w2, saidplurality of π-phase shifting elements defining a critical halftoneperiod which equals w1+w2.
 28. The photolithography mask of claim 27,wherein said mask is utilized in a photolithography process comprisingan exposure source having an associated wavelength λ and an objectivelens having an associated numerical aperture NAo.
 29. Thephotolithography mask of claim 28, wherein said critical halftone periodof said non-resolvable π-phase shifting elements is less than 0.61multiplied by (λ/NAo).
 30. The photolithography mask of claim 26,further comprising scattering bars adjacent any isolated π-phaseshifting elements, each of said scattering bars having a widthsufficiently narrow such that none of the individual scattering bars areresolvable.
 31. The photolithography mask of claim 30, wherein thedistance from a scattering bar to an isolated π-phase shifting elementequals K_(s)(λ/NAo), λ equaling the wavelength of the exposure source,NAo equaling the numerical aperture of the objective lens of theexposure source, and K_(s) ranging from 0.55 to 0.70.
 32. Thephotolithography mask of claim 30, wherein the width of each scatteringbar equals K_(w)(λ/NAo), λ equaling the wavelength of the exposuresource, NAo equaling the numerical aperture of the objective lens of theexposure source, and K_(w) ranging from 0.20 to 0.25.